1. Field of the Invention
The present invention relates to automatic flight control systems for aircraft and, more particularly, to dual channel control systems having improved feedback means and improved monitoring means to substantially eliminate output transients due to failure of one of the channels.
2. Description of the Prior Art
Dual channel servo actuator systems having their outputs driving a common load through a differential gearing are known in the prior art and are exemplified by the systems described in applicant's assignee's pending U.S. patent application Ser. No. 811,653 entitled "Fail Passive Dual Servo with Continuous Motor Speed and Acceleration Monitoring" by M. T. DeWalt, filed June 30, 1977 and applicant's assignee's U.S. Pat. Nos. 3,504,248 and 4,035,705, entitled "Dual Channel Servo System Having Torque Equalization" and "Fail Safe Dual Channel Automatic Pilot with Manuever Limiting," respectively, issued on Mar. 31, 1970 and July 12, 1977, respectively, both by H. Miller. A further example of the prior art is U.S. Pat. No. 3,462,662 entitled "Monitoring and Fault Correction System for a Multiple Channel Servo Actuator," which issued on Aug. 19, 1969 in the name of W. E. Carpenter.
These dual channel control systems comprise two servo channels each including an electric servomotor driven by a servo amplifier to provide first and second outputs to the inputs of a differential gear, the output of which drives the output load or control surface. Position sensor and tachometer generators are coupled to the output of the differential gear and to the motor shaft, respectively, to provide position and rate feedback signals to the input of the associated servo amplifier. The torque transfer characteristics of the differential are such that movement of the output member or control surface caused by a failure in one of the channels is prevented or minimized by movement of the output of the other or "good" servo channel, thereby minimizing undesired transient maneuvers of the craft in response to such failure. The desired characteristic of the differential gear velocity summing mechanism is to cause a failed servo to back drive the good servo in an opposite direction thereby resulting in zero output movement. This characteristic will be provided inherently by the differential gear mechanism only if the output load resistance is greater than the resistance of the good servomotor. The output load member is made to appear stiffer by high gain position feedback derived from the output side of the differential summing mechanism. The position sensor of the "good" channel must respond to a control surface output caused by the failed channel such that it activates its servo channel's motor in a reverse sense such that the resultant output to the control surface is minimized. Thus, a failure in order to be compensated by the dual channel system, must necessarily propagate some disturbance to the control surface or load member. In addition, it is noted that the "good" channel's rate feedback means, i.e., its tachometer, normally provides a damping function which necessarily opposes the "good" motor's speed build-up during compensation of the failure. Accordingly, it is noted that although various monitoring systems are included in the prior art to detect a failure and disengage both channels for fail-passive operation or to isolate a failure and brake or clamp the failed channel, a failure in a channel may propagate a substantial undesired output to the control surface before the "good" channel comes up to speed and before the monitoring systems may react to clamp or brake the failed channel. At cruise altitudes, such a disturbance may be inconsequential. However, certain flight conditions, for example, during a final approach or landing flare-out maneuver, such a disturbance may produce a serious flight path departure and jeopardize a safe landing.
Accordingly, it is desirable to provide a control system with means to rapidly compensate for a servo channel failure such that substantially no surface output or attitude transient is developed. In addition, it is desirable to provide a fault isolation capability to isolate the failed servo channel and insure the integrity of the good control system in a fail operational manner.